A large repertoire of assay methods are available for determining the presence of naturally occurring or synthetic molecules in biological fluids. Thus it is commonplace in a commercial or academic laboratory for one to employ a battery of cytochemical, immunochemical, radiochemical and physical chemical assays, or variations thereof, where detection of the substance of interest is premised on specific recognition by antibody, enzymatic reactions, viscosity changes, etc. Particularly popular are immunochemical assays reliant on antibody/antigen complex formation, and either radiometric or colorimetric detection of the complex.
Associated with each type of assay are advantages and disadvantages that are uniquely characteristic of that particular assay. For instance, radiochemical-based assays exhibit high sensitivity, yet present significant handling and storage problems. In contrast, the opposite is generally true for nonradiochemical-based assays; that is, they are less sensitive but do not have the handling and storage problems of radiochemical assays. Because of the problems associated with radiochemical assays, considerable effort has been expended in order to develop assays as sensitive as radiochemical assays but without their drawbacks.
Bioluminescent assays theoretically offer the sensitivity of radiochemical assays but without the attendant problems; however, for several reasons they have not been widely utilized. Most such assays are based on light emission effected by the catalytic activity of luciferase with luciferin in the presence of ATP and molecular oxygen. The reaction consists of the conversion of luciferin to oxyluciferin with the concomitant generation of light. Thus, in lieu of using radiotracers, luciferase can be covalently attached to a ligand and then employed in any number of competitive binding assays. Nearly all bioluminescent assays employ luciferase isolated from either insects or bacteria, and luciferase from the bacterium Vibrio fischeri has recently been cloned. This bacterial luciferase consists of two different subunits. Because of its higher quantum efficiency and single subunit structure, insect luciferase is preferred over the multisubunit bacterial luciferase.
Unfortunately, there are at least two problems associated with bioluminescent assays. First, luciferase is not readily available and easily isolated; and, second, the enzyme is easily denatured and hence rendered inactive by the chemical reactions necessary to effect covalent attachment to a ligand.